Bacterial blastIdentification tip: Infection starts as black
lesions in the leaf petiole and progresses into leaf axils.
Leaf blades curl, dry, and drop prematurely, often
leaving petioles remaining stuck on the twig. When twig
lesion girdles the stem, twig and branch dieback can result.
Occurs mostly in the Sacramento Valley on the exposed side
of trees facing wind-driven rain.

Botrytis rotIdentification tip: Botrytis fungus can infect
and kill young shoots (shown here on plum), Botrytis damage
to citrus is uncommon in California, but sometimes develops
after prolonged rainy conditions. especially in coastal
lemons where infections can move from old blooms to developing
new lemons in spring.

Wind damageIdentification tip: Scattered dead twigs can develop on trees after
hot dry winds. Wind damage is more prevalent on the more exposed trees, such
as trees along borders, and if soil is dry or roots are unhealthy. Bacterial
blast, Dothiorella blight, and certain abiotic disorders (frost and wind) cause
similar damage. Bacterial blast can also look similar
to this by April when twig lesions have girdled the stems.

AnthracnoseIdentification tip: Twig dieback, premature leaf drop, and dark staining
on fruit can occur after infection by fungi including Colletotrichum.
Dying leaves and twigs become covered with dark fungal spores by which the pathogen
spreads. Anthracnose is more common after a prolonged wet spring.

Phytotoxicity from glyphosateIdentification tip: Small, puckered, needlelike leaves; elongate,
willowlike foliage on new shoot growth; fewer leaves than normal; and buds that
only partially open can occur after trees are contacted by glyphosate herbicide.
Injury is typically confined to exposed branches on the lower outer canopy.

Growth regulator injuryIdentification tip: Leaves cup or curl and
become yellowish if 2,4-D is incorrectly applied to manage
fruit drop and fruit size. Plant growth can be stunted.
Pistils may turn brown and necrotic on young fruit.

Nitrogen deficiencyIdentification tip: Foliage is pale overall when nitrogen is deficient.
The causes include a true deficiency of nitrogen, adverse soil conditions, or
unhealthy roots.

Phytotoxicity from diuronIdentification tip: Leaf veins can turn yellow or white (vein clearing)
after herbicide application due to citrus root uptake of bromacil or diuron.
After simazine exposure, leaves yellow between veins with the chlorosis increasing
in severity in proportion to the amount of herbicide exposure.

Mesophyll collapseIdentification tip: Soft tissue between leaf veins becomes sunken
and translucent or pale due to moisture stress when trees are unable to provide
leaves with enough water; affected leaf tissue then becomes tan-colored or necrotic.

Zinc deficiencyIdentification tip: Extensive chlorosis develops between veins when
zinc is severely deficient; leaves may be smaller in size on shoots that have
shortened internodes.

Phytotoxicity from simazineIdentification tip: Pale blotches develop in leaves after exposure
to simazine herbicide. Interveinal chlorosis can range from relatively mild to
severe depending on the amount of herbicide absorbed.

Citrus
greening (Huanglongbing) (868
KB, PDF)Identification tip: Leaf mottling and yellowing
that crosses leaf veins helps to distinguish citrus greening; yellowing in
just one section of a tree is characteristic of citrus
greening. The cause is a bacterium spread by aphidlike
psyllids. Symptoms include stunted trees, leaf and fruit
drop, twig dieback, and fruit that are lopsided, small
and bitter-tasting. When zinc deficiency is the cause discoloring
occurs between distinctly greener veins. Report suspected
citrus greening to agricultural officials if found in California.

Manganese deficiencyIdentification tip: Leaves turn yellowish overall but larger veins
remain slightly green where manganese is deficient. Zinc deficiency symptoms
are somewhat similar to those of manganese deficiency. Both are more prevalent
on young leaves of the fall growth flush as soils cool and root activity diminishes.

Phytotoxicity from oil and copperIdentification tip: Defoliation of the entire tree occurred when
a sulfur-containing fungicide was applied after oil spray for scale control.
Fruit drop, branch dieback, and discoloring or "water spotting" of
fruit are other damage symptoms from oil misapplication.

Sodium toxicityIdentification tip: Yellowing, bronzing,
or browning of leaves and premature foliage drop can
be due to excess sodium. High overall salinity or an
excess of either sodium or chlorine can cause this
leaf damage or general symptoms throughout the tree
resembling drought stress, such as stunted growth.

Boron toxicityIdentification tip: Leaf mottling or yellowing,
spotting on the underside of leaves, and premature
leaf drop can occur from excess boron; severe symptoms
can include twig dieback.

Hail damageIdentification tip: Small scars on twigs or fruit all occur on the
upper or outer exposed side of branches injured by hail. Wounded tissue is sunken,
leaves are torn and shredded, and may drop prematurely after hail impact.

Sunburn damageIdentification tip: Leaves with chlorotic or necrotic spots and fruit
with yellow to brownish, leathery areas occur mostly in the south and west canopy
sides when sunburn is the cause.

Citrus
canker (Bacterial canker) (636
KB, PDF)Identification tip: Circular, scabby lesions
develop on fruit, both sides of leaves, and on twigs. Lesions
on fruit and leaves are surrounded by a dark or water-soaked
margin and yellowish halo. Citrus canker lesions are raised,
unlike the sunken twig scars from hail injury or the roundish
discoloring on fruit from cold and sunburn. Report to agricultural
officials this exotic disease if found in California.

Photo not available.

Citrus nematodeIdentification tip: Reduced tree and fruit
size, twig dieback, and decline in growth can be caused
by root-infecting nematodes. Diagnosing this malady entails
ruling out other causes, inspecting roots, and sending
root and soil samples to a diagnostic laboratory.

Phytophthora root rotIdentification tip: The pale green to yellowish
leaves on the left tree can have several causes, including
root nematodes and diseases such as Armillaria root rot
and Tristeza. Shown here is Phytophthora root rot on avocado.

Dothiorella blightIdentification tip: Leaves and twigs in a group may decline and die,
either on scattered branches or overall on the entire tree. Fruit and leaves
may remain attached on dead branches. Bark may discolor and exude gum. Cutting
into cankers may reveal discolored brown to yellowish cambium. Young trees may
die if affected tissue is not removed.

Tristeza twig diebackIdentification tip: Fruit, leaves, and twigs
may dry and die on infected trees. Citrus may die suddenly
or after a slow decline. Other infected trees may recover
from symptoms. Tristeza occurs primarily on old trees where
sweet orange scion is grafted to a sour orange rootstock. Armillaria, Phytophthora,
and red scale are among the more common causes of dieback.

Citrus greeningIdentification tip: Yellowing in just one section
of a tree is characteristic of citrus greening. The cause
is a bacterium spread by aphidlike psyllids. Symptoms include
stunted trees, leaf and fruit drop, twig dieback and fruit
that are lopsided, small and bitter-tasting. Report to agricultural
officials this exotic disease if found in California.

Stubborn diseaseIdentification tip: Young trees affected by
stubborn disease are stunted and produce undersized and
few fruit. Off-season growth flush or bloom and upright
leaf growth are other symptoms of this disease caused by
a phytoplasma, Spiroplasma citri.

ExocortisIdentification tip: Reduced tree size and low
yields in older trees are characteristic of infection by Exocortis
viroid. Examining around the root crown will usually
reveal cracked and peeling bark when Exocortis is the cause.

FrostIdentification tip: Cold weather may kill only outer foliage
and leaf tips or cause virtually all foliage on the tree
to turn brown and die. Cold-damaged fruit may drop.

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